The present invention relates to an image transfer device for transferring a toner image formed by the electrophotographic process, etc., onto a copying material such as a sheet of copying paper, and more particularly to an image transfer device suitable for reproducing a multicolor image by superposing different color toner images on a same sheet of copying material through a series of toner transfer operations.
Various methods and devices therefor have hitherto been proposed for the purpose of obtaining multicolor image copies, using the electrophotographic process. The most commonly used method for obtaining such multicolor image copies is such that on an image carrier is formed a first color separation latent image to be developed by a first corresponding color toner, and the toner image is then transferred onto a copying material such as a sheet of paper, and on this are superposed sequentially a plurality of different color toner images by repeating the same process for the second, third . . . transfers according to a necessary number of color separations. In the process of superposing sequentially different color images on a copying material, the perfect register of the copying material to the image carrier is very important, and if the register is inaccurate, a doubling trouble occurs, and as a result the thus produced image becomes useless.
As means for accurately regulating the location of a sheet of copying paper to the image carrier, a transfer device of the type of the following construction is mostly used: A transfer drum is arranged adjacently to and in contact with an image carrier, and the drum, which has a sheet of copying material fixed thereto, is rotated synchronously with the image carrier, the copying material keeping always a constant positional relation with the image carrier. The transfer drum usually has thereon a mechanical detention means (gripper) to automatically stop and fix thereto the leading end of a sheet of copying material that has been brought by feed rollers, etc.
In transferring the toner image on the image carrier onto a sheet of copying material, in order to move the toner onto the copying material, an electrostatic process for charging the copying material is required, and in addition, the image carrier itself has electric charge. For these and other reasons complex electrostatic attractions are produced to act upon the copying material. The copying material, also because it is subjected to the mechanical force from the image carrier or transfer drum, tends to slip out of place. For the above reasons, it is the status quo that the mechanical detention of the copying material is adopted as the most secure way to prevent the copying material from the slippage.
For example, U.S. Pat. No. 3,729,311 discloses a color copier comprising a copying material-holding drum which is in contact with an image carrier drum, the copying material-holding drum having a gripper for fixing a copying material thereto. Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 18653/1980 discloses a color copier comprising a support for a copying material, the support being comprised of an insulating surface-having mesh screen in contact with an image carrier. The instance in this publication describes that mechanical means for detaining a copying material is not necessarily required because the copying material is electrostatically attracted to the support therefor, but the screen-type support's contact area with the copying material is small, so that in order to insure the fixing and retention of the copying material, it is necessary to provide a detention means to the support.
However, in the case where a detention means is provided on the transfer drum, there arise many such problems that a complex mechanism is required for the automatic detention of a copying material, release of the detention and separation of the copying material; restrictions are put on the rate and sequence of the copying operation due to waiting for the detaining position; the detention section is soiled, and blank space is produced at the copying material's leading end detained by the detention section; the cleaning device for the copying material-holding drum requires a mechanism for eluding the gripper position; the gripper's soil by toner soils the copying material; and the like. Accordingly, the development of a more simplified and secure copying material-fixing means has been demanded.
It is therefore an object of the present invention to provide an image transfer device for multicolor electrostatic recording apparatus, the image transfer device being of a simple structure capable of securely transferring and superposing a plurality of different color toner images without doubling onto a copying material.